1. Field of the Invention
The present invention relates to a printing method of measuring color densities of patches included in a control strip printed on a printed matter and inspecting or managing the printing quality, the printed material and a printing control device.
2. Description of the Related Art
To inspect or manage the printing quality, there is a printing method of printing a control strip for measuring the printing quality on a printed matter, measuring color densities of patches (color patch) included in the control strip, and performing printing control based on the measured color densities.
In the case of the printing method, an ink amount is generally inspected with solid patches of four colors such as K (black), C (cyan), M (magenta), and Y (yellow) which are basic colors of printing.
In this printing method, however, the following are not inspected: fluctuation of the dot gain value of a halftone dot to be printed, doubling in which a dot is multiply printed, and slur caused by friction of a dot. Therefore, when using this printing method, it is difficult to assure the quality of every color in a printed image.
As a printing method of solving the above problem, there is a method of inspecting or managing the printing quality by using a control strip including not only solid patches of K, C, M, and Y for inspecting an ink amount but also patches of K, C, M, and Y for inspecting the fluctuation of dot gains of K, C, M and Y.
The printing method using the control strip requires a space for the control strip on a printing sheet.
Therefore, the printing method using the control strip tends to be used for a sheet-fed printing which makes it easy to secure the space rather than an web offset printing which makes it difficult to secure the space for the control strip.
The printing method using the control strip is not frequently used for the web offset printing because of the above reason. However, because higher-quality printing is requested, management of the printing quality using the control strip is also studied on the web offset printing.
To apply the printing method using a control strip to the web offset printing, some systems are developed which use the slender control strip in a narrow space on a printing sheet.
In the case of offset printing, it is requested that patches to be measured printed on an a printed matter is thin and small. However, to accurately measure a color density, it is necessary to reduce that the color density of a patch to be measured is influenced by the color density of an adjacent another patch. Therefore, it is necessary to consider a resolution which can be measured by a measuring device for measuring a patch.
In general, an ink amount for offset printing is adjusted by several ink keys arranged in the direction perpendicular to the running direction of a printing sheet.
Ink amount is adjusted in accordance with the opening degree of blades divided by the number of inks to be used for printing.
When the running direction of a printing sheet does not correspond between a printed pattern and patches, it is impossible to obtain control information which is used to print the pattern.
Using an inspection method including a gray patch in which is a screen tint of three colors such as C, M, and Y in addition to patches of four colors such as K, C, M, and Y which are basic colors for printing, the quantity of information of the image to be printed is more than the case of using a method of performing an inspection with only ink amount of four colors such as K, C, M, and Y, and as a result, the quality of printed matter is stabilized.
This is because the control information on printing qualities (for example, balance of amount of inks of three colors such as C, M, and Y, dot gain, contrast, and trapping), which cannot be obtained from the inspection of quantities of amount of four colors such as K, C, M, and Y which are basic colors of printing, can be obtained from the inspection using the gray patch.
Among the above printing qualities, it is difficult to control trapping by adjusting normal ink or water during press running. Therefore, detection of trapping is generally used to check the state of a material or press machine.
Because most images to be placed on a publication printed matter or commercial printed matter are expressed by dots or multiplication of dots of two colors or more, dot gain and contrast greatly influence the printing quality.
Document 1 (U.S. Pat. No. 4,852,485) discloses a method of controlling inks of an offset printing machine in accordance with the data obtained by measuring patches of control strips printed on a printed matter.
In Document 1, a control strip includes at least one screen patch (screen-tint patch) and one solid patch each color zone (ink key zone). As one example, a screen patch at halftone-dot area rates of 60% for C, 50% for M, and 50% for Y is disclosed. Moreover, as another example, a screen patch at halftone-dot area of 50% for C, 41% for M, and 41% for Y is disclosed. Furthermore, a combination patch (3C gray) is included in the control strip. Document 1 discloses that screen tints at halftone-dot area rates of 25%, 50%, and 75% are used. Ink control is performed by using the calorimetric value of single color patches. The colorimetric value of the combination patch is compared with values in a color table and used for control target setting and determination.
Document 2 (U.S. Pat. No. 6,142,078) also discloses a method of controlling inks of an offset printing machine same as that disclosed in Document 1.
Document 2 does not disclose any patch included in a color bar (control strip). In Document 2, the calorimetric value of solid color patches are used for control.
Document 3 (Jpn. Pat. Appln. KOKAI Publication No. 2001-353851) discloses a method of calculating an optimum screen-tint area rate for a printing-quality control method using a screen-tint patch.
Document 3 discloses that it is preferable to keep the halftone-dot area rate of a screen-tint patch between 76 and 86%. Document 3 also discloses that screen-tint patches at a halftone-dot area rate of 76 to 86% should be included in a control strip.
A method of independently controlling each color by using single color solid patches of K, C, M, and Y has an advantage that a response speed until the color density of each color approaches target values is raised because control process is simple. However, because of the control for each color, it may take a long time until a printed matter having a quality as a commercial product is obtained or color balance may collapse in the case of an actual image expressed by a gray scale which is a combination of three colors.
In a method of including a gray patch obtained from screen tints of three colors such as C, M, and Y together in a control strip and performing control by using the gray patch, the quantity of information on a image to be printed increases compared to a method of inspecting only ink amount of four colors such as K, C, M, and Y which are basic colors for printing and performing control, and as a result, the printing quality is further stabilized. However, because the quantity of information is too much, a response speed tends to delay.
Because the balance between amount of inks of three colors such as C, M, and Y influences the hues reproduced by three colors, it greatly influences the printing quality.
By noticing the above point, Document 4 (Jpn. Pat. Appln. KOKAI Publication No. 2001-80052) discloses the invention for obtaining a printed matter close to a color sample by performing the control considering the balance between three colors such as C, M, and Y while measuring each single color patch.